1. Field of the Invention
Embodiments of the disclosure relate to a display device capable of driving at low speed.
2. Discussion of the Related Art
Display devices have been used in various display units, such as portable information devices, office devices, computers, and televisions.
Methods for reducing power consumption of the display device include low speed driving technology. The low speed driving technology is to change a frame frequency (e.g., a driving frequency) based on a change in the amount of data. In a stop image, in which there is no change of data, the low speed driving technology refreshes the screen of the display device using a frame frequency less than an input frame frequency (for example, a normal frame frequency of 60 Hz). In a moving image, in which there is a change in data, the screen of the display device is refreshed using a normal driving method based on the input frame frequency. The display device may change the frame frequency in response to a panel self refresh (PSR) control signal received from a system. For example, when the PSR control signal is input at an on-level in conformity with the stop image, the display device may reduce the frame frequency to a frequency less than 60 Hz. Further, when the PSR control signal is input at an off-level in conformity with the moving image, the display device may keep the frame frequency at 60 Hz.
The low speed driving technology may be implemented through an interlaced driving scheme. In the interlaced low speed driving scheme, one frame is time-divided into a plurality of sub-frames, and gate lines are interlace-driven in each sub-frame. In the interlaced driving scheme, as the number of sub-frames increases, a length of one frame increases. Hence, the frame frequency is reduced. As the frame frequency gradually decreases from 60 Hz for the low speed drive, a data transition frequency (used in the supply of a data voltage) of a source driver decreases. Hence, power consumption is reduced.
However, as described above, the display device capable of driving at low speed may go through a momentary screen flicker (e.g., a glitch phenomenon) generated when the frame frequency changes. The glitch phenomenon is generated when the frame frequency changes for a normal drive during the interlaced low speed drive. As an example of the glitch phenomenon, FIG. 1 shows the glitch phenomenon generated when the frame frequency changes to 60 Hz during the 30 Hz interlaced low speed drive.
FIG. 2 shows a luminance change (e.g., a transient increase in a mean luminance level) measured by a photodiode when the frame frequency changes from 30 Hz to 60 Hz. As shown in FIG. 2, there is a luminance difference between an Nth frame driven at 30 Hz and (N+1)th frame driven at 60 Hz.
A provoking cause of the glitch phenomenon is described below with reference to FIG. 3.
In 30 Hz interlaced driving state, during a first sub-frame SF1 of the Nth frame, odd-numbered display lines L#1, L#3, L#5, and L#7 are sequentially scanned and are charged to a new data voltage, and even-numbered display lines L#2, L#4, L#6, and L#8 are not scanned and are held at a previously charged data voltage. During a second sub-frame SF2 of the Nth frame, the even-numbered display lines L#2, L#4, L#6, and L#8 are sequentially scanned and are charged to a new data voltage, and the odd-numbered display lines L#1, L#3, L#5, and L#7 are not scanned and are held at a previously charged data voltage. In 60 Hz normal driving state, during one frame, all of the display lines L#1 to L#8 are sequentially scanned and are charged to a new data voltage. A polarity of the data voltage charged to the corresponding display line is inverted when the corresponding display line is scanned in a cycle of one frame period.
In this state, as shown in FIG. 3, when the PSR control signal is input at the off-level between the first sub-frame SF1 and the second sub-frame SF2 of the Nth frame, a related art display device omits the second sub-frame SF2 of the Nth frame and immediately changes the frame frequency from 30 Hz to 60 Hz. In other words, the related art display device scans the odd-numbered display lines L#1, L#3, L#5, and L#7 in the 30 Hz driving state and then immediately changes the frame frequency from 30 Hz to 60 Hz in response to the PSR control signal of the off-level. Hence, in the related art display device, when the frame frequency is changed to 60 Hz, a polarity repeat phenomenon is generated in the even-numbered display lines L#2, L#4, L#6, and L#8. A charge amount of the display lines, on which the same polarity pattern is repeated, is more than a charge amount of the display lines, on which the polarity pattern is inverted, with respect to the same data voltage. Thus, in the related art display device, when the frame frequency is changed to 60 Hz, an entire luminance increases because of some display lines, in which the polarity repeat phenomenon is generated. This is perceived as the glitch phenomenon.